When comparing CanJS vs React, the Slant community recommends React for most people. In the question“What are the best client-side JavaScript MV* frameworks?”React is ranked 9th while CanJS is ranked 13th. The most important reason people chose React is:

Since every single UI component is created independently in JavaScript, it becomes very easy to reuse them throughout your app without having to re-write them.

Pros

Pro

Fast rendering

Pro

Compact and easy syntax

Pro

Easy to reuse components

Since every single UI component is created independently in JavaScript, it becomes very easy to reuse them throughout your app without having to re-write them.

Pro

One-way data flow

React's one-way data binding (or one-way data flow) means that it's easy to see where and how your UI is updated and where you need to make changes. It's also very easy to keep everything modular, fast and well-organized.

Pro

Server side rendering

React can render it's components and data server side, then it sends those components as HTML to the browser.

This ensures faster initial loading time and SEO friendliness out of the box, since it's indexed as any other static website by search engines.

Pro

Virtual DOM support

Instead of relying on the DOM, React implements a virtual DOM from scratch, allowing it to calculate precisely what needs to be patched during the next screen refresh. This is orders of magnitude faster than fiddling with the DOM itself.

Pro

Easy to write tests

Since React's virtual DOM system is implemented completely in JavaScript, it's very easy to write UI test cases.

Pro

Supported by Facebook and Instagram

React is built by Facebook engineers initially to be used only for their inner projects especially to solve the problem of building large complex applications with constantly changing data.

Pro

Can be used with different libraries

ReactJS can be used independently as the only library for building the front-end, or it can be used alongside JavaScript libraries such as jQuery, or even Angular.

Pro

Template engine independent

React provides a template engine (JSX) which is easy to use. But it's not mandatory.

Pro

Functional programming style leads to less buggy UIs

Pro

Good debugging tools

React has an official Chrome Extension which is used as a developing and debugging tool. It can be used to quickly and painlessly debug your application or view the whole application structure as it's rendered.

Pro

Extensive SVG support

Since React v0.15, SVG is fully supported. React supports all SVG attributes that are recognized by today's browsers.

Pro

Keep control over your app's logic

React is just a view library, so you still have (almost) full control over how you app behaves.

Pro

Supported by ClojureScript libraries

Reagent, Om, Rum, etc.

Pro

Flux architecture pattern

Flux is a platform agnostic pattern which can technically be used with any application or programming language.

One of Flux' main features is that it enforces uni-directional data flow which means that views do not change the data directly.

With React this is useful because this way it's easier to understand an application as it starts getting more complicated. By having two-way data binding, lead to unpredictable changes, where changing one model's data would end up updating another model. By using the Flux architecture, this can be avoided.

Pro

Tested on Facebook itself

React is used on one of the most visited websites on the planet, Facebook. With stellar results and with millions of people experiencing it every day.

Cons

Con

Hard to find resources outside of the official ones

Since CanJS is not very popular, it can be hard to find learning resources, tutorials, guides or any other resource that may help development.

Con

Heavy on memory

React's virtual DOM is fast, but it requires storing elements in the virtual and real DOM increasing memory usage for the page. This can be a real problem for single-page webapps designed to be left running in the background.

Con

Verbose

React gets a little verbose as applications get more complicated with more components. It's simply not as straightforward as simply writing HTML and JavaScript would be.

Con

Not a complete solution

React does not do everything for the developer, it's merely a tool for building the UI of a web app. It does not have support for routing or models, at least not out of the box. While some missing features can be added through libraries, to start using React and use it in production, you still would need to have experience, or at least a good grasp on what the best libraries to use would be.

Con

Large file size

React's react.min.js is 145.5KB in size. It's much larger than some other libraries that offer roughly the same features and it's almost the same size as some MV* frameworks such as Angular or Ember that offer more features out of the box.

Although, it should be mentioned that sometimes having a smaller library may force developers to reinvent the wheel and write inefficient implementations on features that React already has. Ending up with a larger application that's harder to maintain and/or that has bad performance.

Con

You have to learn a new syntax

Requires to learn a custom syntax, JSX that has some gotchas and introduce complexity, a steeper learning curve, and incompatibility with other tools.

Though you can opt out from JSX and use vanilla JS instead. But that is not recommended since it adds a lot of unneeded complexity which JSX tries to avoid.

Con

Template(view) mixed into code

Con

No support for legacy browsers

React has recently dropped support for Internet Explorer 8. While the library may still work on IE8, issues that affect only IE8 will not be prioritized and/or solved.